Multi-user multiple-input multiple-output (MIMO) transmission using a Grid-of-Beams (GoB) approach has been shown to be an attractive scheme for the downlink for emerging wireless systems. See e.g., IST-4-027756 WINNER II Deliverable D4.7.3, “Smart antenna based interference mitigation,” June 2007. In the GoB scheme, a grid of beams is created by using closely-spaced antennas at the base stations. Independent data streams are transmitted to terminals in geographical locations served by non-overlapping beams. A hallmark of this scheme is that it requires very little channel state information at the transmitter (CSIT), i.e. beam selection.
The grid of beams approach relies on fixed beams. Beams can be steered by means of baseband signal processing, providing improved coverage and less interference. The problem of joint adaptive beamforming from a multi-antenna base station to multiple single-antenna terminals has been solved. See e.g., M. Schubert and H. Boche, “Solution of the multi-user beamforming problem with individual SINR constraints,” IEEE Trans. VT, vol. 53, no. 1, January 2004 (“Schubert”). The beamformers and transmission powers are jointly adjusted to fulfill individual signal-to-interference-plus-noise ratio (SINR) requirements at the terminals. An algorithm is derived that maximizes the jointly-achievable SINR margin (over the SINR requirements) under sum transmission power constraint. A second algorithm is also derived that minimizes the sum transmission power while satisfying the set of SINR requirements. The algorithms require statistical information about the channel models at the transmitter.
A distributed antenna system (DAS) architecture is being considered for IMT-Advanced systems. DAS differs from a conventional cellular architecture in that DAS cells are connected to a central processing unit (CPU) by means of a fast backhaul. Compared to a conventional cellular network, very high spectral efficiency is possible in a DAS network due to coherently-coordinated transmission from DAS cells in the downlink and joint reception at DAS cells in the uplink. However, coherently-coordinated transmission from DAS cells generally requires a large amount of CSIT which overburdens spectral resources.
A solution to this problem has been suggested where an approach similar to the multi-user beamforming approach of Schubert has been adapted for a DAS network with multi-antenna transmission points (TP). The scheme only requires statistical information about the channel models at the TPs.
In the suggested solution, the beamformers and transmission powers are iteratively determined to minimize the sum transmission power while fulfilling the SINR requirements. A problem with this approach is that the feasibility of the solution is not verified beforehand. Due to this, the iterative algorithm may result in an infeasible power allocation and may not converge. This problem has been addressed in Schubert by: first, finding the beamformers and power allocation that maximize the jointly-achievable SINR margin under sum transmission power constraint; second, determining if the SINR requirements are jointly achievable (SINR margin greater than unity); and third, finding the beamformers and power allocation that minimize the sum transmission power while fulfilling the SINR requirements. The solution that minimizes the sum transmission power can be used in a DAS network if the maximum transmission power is less than the corresponding individual power constraint. On the other hand, if the maximum transmission power is higher than the individual power constraint, the solution is considered infeasible.
A problem with the solution suggested in Schubert is that the sum transmission power constraint, although suitable for single-cell transmission, is not very suitable for multi-cell transmission in a DAS network as each TP antenna is individually power-limited. The maximum jointly-achievable SINR margin obtained under sum power constraint is not meaningful for a DAS network.